Circulating levels of the steroid hormone, progesterone (P), increase during development of the primate corpus luteum (CL) and then decline during luteal regression unless chorionic gonadotropin (CG) rescues the CL in early pregnancy. To determine if these changes in P levels are related to steroidogenic enzyme expression, the enzymes converting cholesterol to pregnenolone (P450SCC) and then to progesterone (3 -HSD) were detected by immunohistochemistry in macaque luteal tissue throughout the menstrual cycle and simulated early pregnancy. CL were collected from rhesus monkeys (n=3/group) in the early (day 2-3 post-LH surge), mid (day 7-8), mid-late (day 10-12), and late (day 14-15) luteal phase and after 1, 3, 6, or 9 days of hCG (Profasi, Serono) treatment beginning on day 9 of the luteal phase. Tissues were fixed in 4% paraformaldehyde, embedded in OCT and frozen on dry ice or fresh-frozen in liquid propane. Specific cytoplasmic staining for P450SCC and 3 -HSD was present in luteal cells, but not vascular elements, within all luteal tissues examined. It was not possible to distinguish between granulosa and thecal lutein cells for either enzyme. In the menstrual cycle, P450SCC staining of individual luteal cells was most intense during the early luteal phase; however, during midluteal phase more cells were labeled. By mid-late and late luteal phase, more luteal cells were only lightly stained. In contrast, 3 -HSD immunostaining was lightest at early luteal phase and darkest at midluteal phase. By mid-late luteal phase, a heterogeneous staining pattern was apparent, with some steroidogenic cells staining intensely and others staining lightly. In simulated early pregnancy, P450SCC immunostaining was greatest on day 1. Thereafter, the number of positively labeled cells and staining intensity progressively declined. 3 -HSD staining was darkest after 1-3 days of hCG exposure. By day 9, steroidogenic cells exhibited a heterogeneous labeling pattern. Thus, somewhat different immunostaining patterns of the steroidogenic enzymes, P450SCC and 3 HSD, were observed, but both varied during the lifespan of the primate corpus luteum in the menstrual cycle and following simulated early pregnancy.